The story
From "The Prisoner of Benda" (2010), season 6, episode 10 of Futurama. The episode and the theorem were written by the mathematician Ken Keeler.
Etymology from Wiktionary, CC BY-SA 4.0.
Futurama theorem
Today it means: A theorem which proves that, given a mind swap machine that can't be used on the same pair of people more than once, it is possible to reverse any number of mind swaps by introducing two more people that did not have their minds swapped with anyone before. Full definition →